A preponderant number of applications which entail the use of coal tar or petroleum pitches require the carbonization of the pitch material. Examples of carbonization processes include coking and graphitization. During the carbonization process, it is normal to lose between 25 and 65% of the binder, the exact loss being dependent on the volatile content of the pitch. The loss of volatiles to the environment is additionally undesirable in that they constitute a source of air pollution. It is common practice in the industry to indicate this weight loss by the fraction of the starting material which remains upon carbonization, eg. the fraction of the material remaining upon coking is called the "coking value" of the pitch. This is particularly important when the pitch is used as a binder in the formation of coked carbon bodies.
Generally a carbon electrode is a two-phase or binary carbon system consisting of a petroleum coke filler (an aggregate with a specific size distribution) whose particles are bound by a pitch coke phase (called binder pitches) which is developed during carbonization of the electrode paste. In aluminum production, the carbon particles desintegrate from the working surface of an anode by selective oxidation of the binder pitch and thus form carbon dust. The binder pitch phase of anodes is consumed preferentially during oxidation, both electrolytically and chemically; this also results in dusting and the net effect is that carbon consumption is significantly more than the stoichiometric requirement sometimes doubling the stoichiometric amount. This is a substantial amount, if one considers that 83-90% of the anode is carbon filled.
Also a problem encountered with baked carbon bodies obtained from pitches is their relatively high oxidation rate, particularly noticeable when they are used as refractory materials or as electrodes. This oxidation rate can be attributed to such factors as the porosity of the carbon body, its specific surface and the inorganic impurities present in the carbon body.
Techniques hitherto employed to decrease this oxidation rate include pressure impregnation of coating of previously baked carbon bodies with aqueous solutions of oxidation retardant materials such as phosphates, silicates, etc., after which the carbon bodies are rebaked to drive away the moisture. While the former technique requires pressure treating equipment and large volumes of an often expensive impregnant, neither technique succeeds in inhibiting oxidation throughout the interior of the carbon body.
British Pat. No. 865,320 teaches adding oxidation inhibitors to the coke filler-pitch mix before baking. This technique however, requires 4 to 20% weight of the corresponding mix, of the additive. This is expensive in comparison to the base material being treated. Such large amounts of additive may have a deleterious effect if the resulting carbon body is used for making an electrode. During electrolysis such additives are normally incombustible and constitute a substantial portion of the electrode which may form residues and contaminate the system.
Also U.S. Pat. No. 4,298,396, dated Nov. 3, 1981 as invented by Limonchik et al, describes systems improving oxidation resistance of anodes in such systems however, dusting is not improved.